This invention relates generally to signal waveform detectors, and more particularly to optical systems which detect signal waveforms.
In fiber optic interferometric sensing systems, phase noise produced by thermal fluctuations or mechanical vibrations is so large that the output is driven non-linear. This non-linearity causes the desired signal to fade in and out. It is known to optimize the signal output by employing a demodulation technique. Prior art demodulation techniques include heterodyne-FM, homodyne phase-lock, phase modulation, phase-swept phase-locked-loop and frequency stabilization. All of these techniques suffer from disadvantages.
A device of the heterodyne demodulation type has been described in the paper "Optical Hydrophone for Sonar", J. A. Bucaro, EASCON 78 Record, IEEE Publications 78 CH 1354-4 AES, p. 298.
In a heterodyne system the light frequency of one beam of the interferometer is shifted in frequency by an amount equal to the frequency used to drive a Bragg modulator. The output of the interferometer detected by a photodetector has a carrier signal at the frequency used to drive the Bragg modulator. If FM discrimination is utilized to obtain the sensor output of interest, heterodyne demodulation is relatively insensitive to intensity and polarization fluctuations. In systems applications heterodyning demodulation can employ one modulator to provide the optical source for several sensors. However, heterodyning-demodulation has significant disadvantages. Integrated optic Bragg modulators restrict the optical power. Either or both optical and electrical power requirements for heterodyne systems are worse than for other detection techniques; minimum detectable phase shifts currently demonstrated with heterodyne demodulation techniques which employ reasonable package sizes are approximately two orders of magnitude greater than other techniques. Heterodyne detection does not appear competitive with alternate demodulation techniques.
A device of the homodyne phase-lock type has been described in the paper "Measurements of Small Phase Shifts Using a Single Mode Optical-Fiber Interferometer", P. A. Jackson et al., Optics Letters, pp. 139-141, April 1980.
In homodyne phase-lock demodulation, the dc output of the detector is utilized as a signal proportional to the instantaneous phase of the interferometer. The output of the detector is amplified, low pass filtered and then sent to a device which induces a phase shift in the interferometer proportional to the applied voltage. In a system where multisensor operation is required, this demodulation technique has the disadvantage that a modulator is required for each sensor.
A device of the phase modulation type has been described in the paper "Fiber Interferometer Demodulation and Noise", J. H. Cole et al., FOC 81 EAST Fiber Optics and Communications Proceedings, published by Information Gatekeepers Inc.
The phase modulation technique eliminates the requirement for a modulator associated with each sensor, but as an amplitude demodulation technique suffers sensitivity to amplitude fluctuations.
A device of the phase-swept phase-locked-loop type has been described in the paper "Accurate Phase Measurement System for a Fiber Optic Interferometer", I. J. Bush, Digest of Technical Papers, Conference on Lasers and Electrooptics, 10-12 June 1981, IEEE/OSA Washington, D.C.
The phase-swept phase-locked-loop as an embodiment of the homodyne detection system also requires a modulator per sensor. This technique, along with the homodyne demodulation scheme, requires modulators with large dynamic ranges.